Bistability of ice giant dynamo action

The morphology of the magnetic fields of Uranus and Neptune are radically different to those of all other planets in our solar system. Despite only having in-situ measurements from one Voyager 2 fly-by of each planet, it is clear that they are not dominated by an axial dipole and are instead multipolar. However, due to the rarity of large-scale multipolar planetary magnetic fields in our solar system, the focus of numerical dynamo studies has primarily been on dipole dominated models. Here, we revisit existing numerical dynamo parameter surveys, selecting cases of particular interest. We focus on multipolar models that are in the regime where rotation dominates over convective turbulence (i.e., the low local Rossby number regime, Ro_ell<1). We have also performed simulations with high Ro_ell to quantitatively disambiguate between the two dynamical regimes by comparing the characteristics of these systems, such as their field strengths, flow characteristics, and spectra. In this comparison we reconsider the best ways to characterise multipolar dynamos. While many of the existing tools sweep them into one vast category of “non-dipolar” models, we find them to display a number of vastly differing properties. Characterisation of these models allows us to assess ice giant-like properties with our existing data as well as predict what higher resolution future measurements of their magnetic fields could reveal.